Have you researched anything about littermate syndrome? Genuinely
Dogs dont need to be related to get it. Its also common in cockers. Often pups get along fine in puppy hood but once adolescence and then maturity hits can develop massive issues with each other. I know of three people in my close circle who had this. One managed to keep them but had to rotate them and limit the time together. Two ended up rehoming one, both after a significant injury to a dog and them for getting in the way
At best you can end up with dogs that dont develop their own individual confidence and skill so cant function independently. Being able to do stuff independently is important because if one dies, or even just gets injured and needs to rest, or develops training needs you'll need to take them out individually
What i saw in rescue other than the actual horrendous fights was also a combo of things. People assuming because there is 2 that they will need less, so sometimes people got two when they shouldn't have got 1 so had behaviour issues from boredom and lack of training. Then there was people who had enough time for 1 but got two so each dog got half the attention it should.
Each dog needs to be given the proper individual training time. In this scenario it's common for a more fearful dog to get over looked for example and dragged into situations by its confident brother. Nobody noticed until the dog is reactive.
Think of things like teaching your dogs proper loose lead walking and dog play etiquette. With two you are always accidentally rewarding pulling, saying yes when one pulls because you are talking to the other. Then you cant keep proper eyes on them, so the standard for behaviour is lower. The dogs then become difficult to walk, so get walked less or in a chaotic way which compounds the leash problem and results in an increase in bad behaviour at home. You dont do the things like socialisation because its too hard to manage with two.
Honestly having two puppies at the same time is a real problem and most rescues end up seeing it over and over again